It is necessary to provide a reliable device for metering the volume of various flowing liquids. It has severely been required to correctly measure the flow of crude oil and various petroleum products such as gasoline and kerosene in view of recent striking price rises, for instance, in oil storing tanks, oil refining plants, fuel filling stations and the like.
The specific gravity and consequently the volume of liquid inclusive of such fuels vary depending on the temperature thereof. For instance, some kerosene has a specific gravity of 0.810 at a temperature of 15.degree. C., 0.820 at 0.degree. C. and 0.803 at 25.degree. C. Supposing that a diesel engine automobile is supplied with such kerosene. Measured by volume, the same amount of fuel costing $50 at 0.degree. C. would cost $51.06 at 25.degree. C., if no compensation is made in the metering device in reference to the supplied fuel temperature at the moment.
Like many countries, the Japanese Industrial Standard prescribes coefficients for compensating for the volume of crude oil and petroleum products having specific gravities ranging from 0.5 to 1.1 at a standard temperature of 15.degree. C., in reference to various temperatures. For instance, in JIS K2250, the specific gravities of crude oil and petroleum products whose flow is to be measured are given in increments of 0.005 and the temperatures to be taken into consideration for determining the coefficient for the compensation are given in increments of 0.5.degree. C.
The compensation is to be made in reference to considerably bulky tables such as JIS K2250. For instance, for kerosene having a specific gravity of 0.810, the coefficient is given as 1.0132 at 0.degree. C. and 0.9912 at 25.degree. C. on the corresponding table. Thus, the nominal volume measured by a meter must be compensatingly multiplied by 1.0132 when the liquid temperature is 0.degree. C. and by 0.9912 when it is 25.degree. C. to make it coincident with the standard volume at 15.degree. C.
Such compensation may be made according to various methods. It is of course possible to calculate using such coefficients in reference to ambient temperature, but this is too annoying and incorrect since there is too much difference between liquid temperature and ambient.
Mechanical automatic compensation has been used fairly widely, in which temperature fluctuation is converted to the mechanical displacement of bellows which adjusts the flow meter. This is far from the recent severe requirement mainly because the volume change rate is not uniform depending on the specific gravity of the liquid.
U.S. Pat. No. 4,101,056 proposes to compensate the metered volume by means of an electronic computer. However, the compensation is made therein by every one program cycle so that it is necessary to calculate volume during this one cycle from the rotation angle of the meter. Furthermore, fairly important changes in characteristics of circuitry elements such as capacitors, resistances and semiconductors, which must be arranged in considerable numbers in amplifiers and analog-digital converters, which changes are caused by deterioration of the elements and ambient temperature fluctuation, are ignored therein. In addition, it is preferable to pay more attention to the compactness and cost of the device.